The present invention relates generally to ion implantation and more particularly to improving the life expectancy of ion implantation sources.
Ion implantation is used in integrated circuit fabrication to introduce controlled amounts of dopants into semiconductors. Doping is accomplished by scanning a beam of dopant ions across a semiconductor wafer. Ions created at an ion source are accelerated through apertures, various electric fields, and a mass-selecting magnet, before being implanted in the semiconductor silicon wafer.
The ion source of an ion implantation system generates ions by introducing electrons into a chamber filled with dopant gas. The electrons are generated by thermionic emission from a resistively-heated filament. Collisions of these electrons with dopant atoms and molecules in the gas result in the creation of an ionized plasma consisting of positive and negative dopant ions. An extraction plate with a negative or positive bias will respectively allow the positive and negative ions to pass through the aperture and out of the ion source as an ion beam.
One major failure mode of ion implantation systems using halide containing source gases is that the breakup product is a dihalide which condenses onto cold surfaces as a sticky, conductive residue. This residue forms on the high voltage components of the ion implanter and causes electrical short circuits in the ion source, which can extinguish the arc required to produce thermionic electrons.
For example, with a germanium implant species, Ge+, one example of an ion source material is germanium tetrafluoride (GeF4) gas. In addition to ionization, the energetic electrons formed by thermionic emission break up the GeF4 molecule into several constituents: germanium difluoride (GeF2), elemental (Ge), and elemental (F). The GeF2 is a sticky, conductive material. Boron trifluoride (BF3) and silicon tetrafluoride (SiF4) are source gases which are also subject to the same problem as GeF4, although to a lesser degree.
A second major failure mode is that the physical and chemical sputtering by halide containing plasma deposits sputtered materials from the arc-chamber walls to the filament, thus coating it and preventing thermionic emission.
A third major cause of failure of ion implantation systems is that the reactive halide gas causes thinning of the filament which leads to filament failures.
Finally, the thinning and coating occur non-uniformly along the filament, resulting in stresses which eventually break the filament. Thinning occurs via both physical sputtering and chemical etching of the filament material, since fluorine is constituent.
Thus, there are four major failure modes for ion implantation systems using halide containing source materials. While the ion source life expectancy for ion implantation systems using non-halide containing source materials is generally around 168 hours, with halide-containing materials, such as GeF4, the ion source life is often as low as 10 hours.
Experts in the field of ion implantation have been consulted in an attempt to increase the mean time between failures (MTBF), or life expectancy of an ion source, but none of the experts has been able to provide a satisfactory solution which would significantly increase the source life time.
The present invention provides a method of increasing the ion source lifetime by infusing the source gas, such as germanium tetrafluoride, with an inert gas, such as argon or xenon. The gas mixture causes inert gas sputtering, which has a cleansing effect inside the plasma ambient. Among other advantages, the invention 1) reduces residue build-up, which causes electrical short circuits to the filament, thereby extinguishing the arc required to produce ions, 2) enhances filament thinning by physical inert gas ion sputtering to counteract the coating failure mechanism, 3) dilutes the ion plasma of reactive halide species in order to reduce the thinning failure mechanism, and 4) reduces stress failures in the filament due to non-uniform thinning and coating.
The present invention further provides a built-in cleansing of the ion implantation arc chamber.
The present invention further provides an optimization of existing ion implanters to generate ions from halide compounds without requiring expensive or esoteric equipment modifications or upgrades.